1. Field of the Invention
The present invention relates to control circuits for a vibrating membrane used in particular as a car horn or as an alarm device.
2. Discussion of the Related Art
FIG. 1 shows a conventional circuit for control of a vibrating membrane. A membrane 17 is placed with respect to a solenoid S to be attracted by this solenoid when it generates an excitation field. The solenoid is disposed in a series circuit including a d.c. power supply E, for example, a car battery, a turn-on switch P, and a circuit breaker B which opens when membrane 17 reaches an extension corresponding, for example, to the membrane position shown in dotted lines and designated as 17xe2x80x2. Turn-on switch P is for example a push-button switch or a switch controlled by an external circuit such as a defect detection circuit for triggering an alarm. The operation of this circuit is the following. In the initial state, circuit breaker B is closed and, as soon as push-button P is actuated, a current flows through solenoid S and attracts the membrane to its position 17xe2x80x2. When the membrane is in position 17xe2x80x2, circuit breaker B opens and the membrane returns to its idle position 17. This is repeated as long as switch P remains closed.
This circuit has the disadvantage that the opening of circuit breaker B occurs when the current in the solenoid is maximum. Such an abrupt opening of a circuit generates cut-off pulses and parasitic radiofrequency signals which can have an adverse effect on circuits linked to the vibrator circuit or neighbor thereto, for example, other electronic circuits of an automobile. Also, in the case of an alarm circuit, if source E corresponds to a rectified supply source, the parasitic signals can reach the main power supply.
The present invention aims at overcoming these disadvantages and at providing a vibrator control circuit generating next to no parasitic signals.
To achieve these and other objects, the present invention provides a control circuit for a vibrating membrane excited by a solenoid in series with a d.c. supply and a controlled switch, comprising a capacitor disposed across a series circuit including the solenoid and the switch; and means for opening the switch in the vicinity of a zero crossing of the current in the inductance.
According to an embodiment of the present invention, the controlled switch is formed of a thyristor disposed between the first terminal of the capacitor and the first terminal of the solenoid, the gate of the thyristor being connected to the first terminal of the capacitor by a zener diode and the gate of the thyristor being connected to its cathode via a resistor, the thyristor intrinsically forming the opening means in the vicinity of a zero crossing of the current.
According to an embodiment of the present invention, the circuit further includes a resistor in series between the supply and the capacitor to set the charge time constant thereof.
The foregoing objects, characteristics and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in connection with the accompanying drawings.